Time-resolved immunofluorescence: a sensitive and specific assay for anti-HIV antibody detection in human sera

We describe a new immunoassay, time-resolved fluoroimmunoassay (TR-FIA), for detection of anti-HIV antibodies in human sera. This method is based on the use of a crude virus preparation coated on a polystyrene microtitre plate and of a swine anti-human IgG labelled with a rare earth metal, europium, as fluorescent label chelated with EDTA derivatives. A light pulse from a xenon lamp (340 nm) was used to excite the label and after a 400 microseconds delay time the emission fluorescence was counted for 400 microseconds at 613 nm. This cycle was repeated 1000 times during the total counting time of 1 s. TR-FIA presents considerable advantages over other techniques: (a) it avoids time-consuming, expensive and hazardous virus purification steps; (b) it excludes the use of radiotracers or substrates with potential health risks to reveal the reaction; (c) it has high sensitivity and specificity. A total of 475 serum specimens were tested by ELISA and by TR-FIA. The proportions of positivity were 29.6% by ELISA versus 26.7% by TR-FIA. The sensitivity of both systems was 100%. The specificity was 87.5% for ELISA, whereas it reached a value of 99.4% for immunofluorimetric assay.